Information relating to quality of service obtained from two-wire lines and their associated termination units in integrated services digital networks usually includes measurements of bit-error rates. Such information is digital in nature, and supplies little intuitive insight into the state of various components associated with the two-wire line. Specifically, from a measured bit-error rate for a two-wire line, particularly if the bit-error rate lies close to a normal range, it is very difficult to make meaningful inferences about the state of the line and how robust it would be if conditions such as temperature and humidity were to change. This is particularly important for technicians who are trying to locate intermittent faults which occur only under rare circumstances.
Probably of more interest to a technician involved in fault-finding and maintenance of such two-wire lines would be a set of analogue figures-of-merit. For a two-wire line terminated at a customer's premises in a network termination unit, and terminated at an exchange in a line termination unit, such a set could, for example, include any of the following: (a) the tolerable increase in two-wire line attenuation, (b) the tolerable decrease in receiver sensitivity in the line termination unit, and (c) the tolerable decrease in receiver sensitivity in the network termination unit.
Equipped with this possible set of figures for the two-wire line, the technician could easily determine whether a fault was present, or was likely to become apparent, in the two-wire line itself, the line termination unit, or the network termination unit.
Thus, it is desirable to provide a test apparatus for integrated services digital networks for providing analogue figures-of-merit which can help a technician to locate faults in two-wire lines and their associated termination units.